Enhanced antitumor effect of alectinib in combination with cyclin-dependent kinase 4/6 inhibitor against RET-fusion–positive non–small cell lung cancer cells

ABSTRACT

Rearranged during transfection (RET) fusion–positive non–small cell lung cancer (NSCLC) accounts for 1% of lung adenocarcinoma. Although small molecule agents with RET kinase inhibitory activity such as alectinib, vandetanib, and cabozantinib have been clinically evaluated in RET-fusion–positive NSCLC, an effective monotherapy regimen has not been established. We explored agents to use in combination with alectinib to enhance the antitumor effect of alectinib against RET-fusion cells.

Cell proliferation under co-treatment with alectinib plus each of six chemotherapeutic agents or six molecularly targeted agents was evaluated in vitro. The combination effect was analyzed by IC₅₀ isobologram and combination index using LC-2/ad and Ba/F3-KIF5B-RET cells. The in vivo combination effect was investigated in a Ba/F3-KIF5B-RET xenograft model. The phosphorylation levels of proteins regulating proliferation were measured by immunoblotting.

Palbociclib, a CDK4/6 inhibitor, showed the greatest synergy against LC-2/ad cells in the isobologram analysis and combination index. This synergistic effect was also observed against Ba/F3-KIF5B-RET cells. Another CDK4/6 inhibitor, abemaciclib, also showed a synergistic effect. In vivo, the combination of alectinib plus palbociclib showed a more enhanced antitumor effect than each single agent in a mouse xenograft model with transplanted Ba/F3-KIF5B-RET cells. This combination suppressed the phosphorylation of S6 and Rb more intensely than did either single agent in both LC-2/ad and Ba/F3-KIF5B-RET cell lines, both in vitro and in vivo.

Combination therapy with alectinib plus the CDK4/6 inhibitor enhanced the antitumor effect against RET-fusion–positive cells in vitro and in vivo.

KEYWORDS:

• Alectinib
• palbociclib
• RET
• CDK4/6
• NSCLC
• IC₅₀ isobologram

Acknowledgments

We thank Kumiko Kondoh and Mirei Kohno (Product Research Department, Chugai Pharmaceutical Co., Ltd.) for technical assistance; Kiyoaki Sakata (Research Division, Chugai Pharmaceutical Co., Ltd.) for instruction on the use of isobolograms; and Dr. Kazushige Mori and Kaori Fujimoto-Ouchi (Product Research Department, Chugai Pharmaceutical Co., Ltd.) for helpful suggestions and comments about this study.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Supplementary material

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